In EP 0 875 087 B1 a varistor material for electrical stress control containing doped zinc oxide (ZnO) filler particles embedded in a polymer matrix is shown. The varistor material is specified such that only round or at least smoothly shaped spheroid particles are used and that a majority of the particles has a maximum dimension of between 5 μm and 100 μm. It is claimed that a favourable nonlinearity is achieved with such filler material and that it is suitable for medium voltage cable terminations and joints.
In FR 2 547 451 a similar varistor material having a majority of doped ZnO filler particles with maximum dimensions larger than 100 μm is disclosed for similar field stress control purposes.
In the EP 1 274 102 A1 a varistor granulate comprising a mixture of varistor particles having different nonlinear current-voltage characteristics is shown.
The invention starts from the state of art as described in EP 0 992 042 (WO 99/56290). The filler material comprises sintered varistor granulate made of doped zinc oxide and having a coarse and a fine fraction of microvaristor particles. The dimensions of the microvaristor particles range from 40 μm to 200 μm, in particular from 90 μm to 160 μm, for the coarse fraction and from 10% to 50% of these values, in particular from 32 μm to 63 μm, for the fine fraction. The fine fraction portion shall be approximately 5 Vol % to 30 Vol % of the coarse fraction portion. This bimodal distribution provides a good density of varistor particles, because all varistor particles are substantially round, consequently the coarse fraction arranges itself in close sphere packing and the fine fraction fills interstices. In addition or alternatively to such a bimodal distribution, the filler comprises electrically conductive particles for improved contacting among the closely packed varistor particles. Thus improved nonlinear electrical properties are achieved, in particular increased nonlinear electrical coefficients, increased power absorption and reduced breakdown field strengths. A third fraction of super-fine microvaristor particles having dimensions e.g. below 32 μm may be added to further improve the nonlinear behaviour. However, the sintered varistor granules contain exclusively coarse and fine particles with predominantly spherical shape that are capable of self-arrangement in a close sphere packing. Therefore, each pair of neighbouring particles forms exclusively one single point contact, independent of whether it is a coarse-coarse, coarse-fine or fine-fine particle contact. This strongly delimits the effective contact area and thus the electrical performance. Furthermore, protruding spinel crystals may form on the surface of the microvaristor particles and may act as spacers between the varistor particles thereby blocking their mutual electrical contact.